1. Field of the Invention
The present invention relates to a detection apparatus that detects a point of transition of a signal level of a signal to be measured.
2. Description of Related Art
Conventionally, in regard to a test of an electronic device such as a semiconductor circuit, there is a test measuring an output signal output from the electronic device and deciding whether the electronic device outputs an output signal according to an expected value. At this time, a testing device that tests the electronic device measures a changed point of time and a changed result of the value of output signal.
The testing device performs a measurement referred to as an edge strobe in order to measure a point of transition of the value of the output signal. The edge strobe measurement technique in which strobes of which phases are sequentially delayed are generated and signal levels of the output signal are sequentially detected in the timing of the generated strobes is known as disclosed, for example, in Japanese Patent Laid-Open No. 2001-356153, particularly page 7, and FIGS. 15 and 16 thereof. In this way, the signal levels of the output signal in a plurality of timings having different phases are detected, the transition points of the value of the output signal are detected, and the changed point of time of the signal level and the changed result of the signal level are measured.
When detecting a signal level in the edge strobe measurement technique, a level comparator receives an output signal and detects the signal level using the strobe as an operation clock. The level comparator compares the signal level of the output signal and a predetermined threshold value in the timing by the strobe, and detects which level the signal level is of an H level or an L level.
A conventional testing device detects the timings of a rising edge and a trailing edge of an output signal in the above-described method. However, when measuring a rising edge and a trailing edge of an output signal in the above-described conventional method, measurement results of both edges can have an error in some cases. For example, even if a rising edge and a trailing edge that varies in the same timing are measured, measurement results of both edges can be different from each other in some cases.
As the reason for the error, a hysteresis characteristic of the level comparator used for the detection of a signal level of an output signal is considered. In other words, since a threshold value to be compared with an output signal in order to detect an H level and a threshold value to be compared with an output signal in order to detect an L level are different, measurement results of both edges can have an error when measuring a timing of the rising edge that varies from an L level to an H level and a timing of the trailing edge that varies from an H level to an L level.
Moreover, as another reason, when the output signal passes through a logic circuit, a buffer, etc. in a testing device, the difference of a propagation delay time between the rising edge and the trailing edge is considered.
In this way, by various reasons, the above-described measurement error occurs. It is desirable to reduce such a measurement error in order to test a device having a faster operation speed. However, even if circuit characteristics in a testing device are adjusted in order to reduce such a measurement error, it is difficult to sufficiently reduce the measurement error and also to make such a correction circuit. Moreover, it is unfavorable in consideration of design cost.